trace-cmd: Handle zeroing of set_ftrace_pid on older kernels
trace-cmd: Handle zeroing of set_ftrace_pid on older kernels

Older kernels require a "-1" be passed into set_ftrace_pid to zero
it out. Newer kernels need just an empty string.

The sad part is that using the wrong one will cause the write to fail.

Signed-off-by: Steven Rostedt <43232e92d70cc7aa53504ad0397085ee47bad87f@goodmis.org>

bigzz/trace-cmd,bigzz/trace-cmd